URL Shortener

instructions:

to run: yarn start or npm start and open http://localhost:3000/.
paste a URL in the form and submit to shorten it.
you can copy the short url using the button.
navigate to the short url to visit the original page.

architecture:

i used firebase as it allows to run this simple app without any backend setup required.
for simplicity, i put all firebase config and api keys in the repo. the database allows unauthorised access for 1 week for testing purposes.
for slug generation i simply used the document ID provided by firebase. this is a 20-character unique string.

possible improvements:

write a custom slug-generation function, which would make the generated urls shorter.
validate URLs — this can be achieved either via custom regex or a 3rd-party library.
add missing tests for Resolver - redirecting and displaying warning for incorrect urls.